The present invention relates to a method of manufacturing a metal pipe having an open end expanded eccentrically with respect to its axis.
A metal pipe with an eccentrically expanded open end has been used as an oil supply pipe for a vehicle fuel or the like. To date, such a metal pipe has been manufactured by bulging an open end of an original metal pipe or by connecting a metal pipe with a squeezed open end to another metal pipe with an expanded open end. However, any of these processes is too complicated, resulting in an increased manufacturing cost. In this regard, a different method has been examined, whereby an original metal pipe is radially expanded at its open end by forcibly inserting a tapered expanding punch.
In a conventional expanding method, an expanding punch is forcibly inserted into an original metal pipe 1 with an open end vertical to its axis, as shown in FIG. 1. The open end is plastically deformed to a coaxially expanded state 2 by insertion of the expanding punch. When an expanding punch tapered at its tip is used, a tapered part 4 is formed between a straight part 3 and the expanded open end 2. Thereafter, another punch, which is held at a position shifted or offset from an axis of the straight part 3, is inserted into the expanded open end 2 so as to form an eccentrically expanded open end 5 off centered from the axis of the straight part 3.
Although the eccentrically expanded part 5 is formed by inserting the punch whose center axis is offset a certain distance from the axis of the straight part 3 in a direction D, a deformation ratio of the original metal pipe 1 is varied along a circumferential direction in response to eccentricity. In short, the wall thickness of the original metal pipe 1 is not reduced so much at a side 7 to be expanded without eccentricity, but the original metal pipe 1 is preferentially stretched at a side 6 to be eccentrically expanded along its circumferential direction with less metal flow from the side 7 to the side 6. Consequently, the eccentrically expanded side 6 is thinned along the circumferential direction. The thin wall causes problems, such as cracking or necking, which intensify as the expanding ratio increases. The partially thinned wall also degrades the mechanical strength of a product.
The present invention provides a metal pipe with an eccentrically expanded open end free from cracks and necking, by formation of a coaxially expanded open end, which is elongated along an axial direction of the metal pipe at a side to be eccentrically expanded longer than the opposite side to be expanded without eccentricity, in prior to an eccentrically expanding step so as to promote metal flow from the former side to the latter side without partial reduction of wall thickness along a circumferential direction.
The present invention provides a new method of manufacturing a metal pipe with an eccentrically expanded open end by two steps of coaxial and eccentric expansion.
A coaxially expanding punch is forcibly inserted into an open end of an original metal pipe, so as to plastically deform the open end to a coaxially expanded state such that a side to be eccentrically expanded is longer than the opposite side to be expanded without eccentricity along an axial direction of said original metal pipe.
After formation of the coaxially expanded open end, the coaxially expanding punch is withdrawn from the metal pipe.
Thereafter, an eccentrically expanding punch, which has a boundary between a conical tip and a cylindrical body inclined with a predetermined angle with a respect to a radial direction of the original metal pipe so that the cylindrical body comes in contact with an inner wall of the coaxially expanded open end at the side to be eccentrically expanded earlier than the opposite side to be expanded without eccentricity, is forcibly inserted into the coaxially expanded open end of the original metal pipe so as to plastically deform the open end to an eccentrically expanded state.
In the coaxially expanding step, a coaxially expanding punch, which has a boundary between a conical tip and a cylindrical body inclined with such an angle that a length of the cylindrical body along an axial direction of the original metal pipe is shorter at the side to be eccentrically expanded than the opposite side to be expanded without eccentricity, may be used. An open end of the original metal pipe is plastically deformed to a coaxially expanded state elongated along its axial direction at a side to be eccentrically expanded as compared with the opposite side to be expanded without eccentricity, by forcible insertion of such the coaxially expanding punch.
Furthermore, when the coaxially expanded open end is worked with an eccentrically expanding punch, which has a boundary between its conical tip and its cylindrical body inclined opposite to inclination of the coaxially expanding punch, metal flow is promoted from the opposite side to be expanded without eccentricity to the side to be eccentrically expanded. Consequently, the open end of the metal pipe is plastically deformed to an eccentrically expanded state without significant reduction of wall thickness along its circumferential direction.